The present invention relates generally to current sources and more specifically to high impedance, high accuracy current sources and their application to current mirrors and differential to single-ended current converters.
Ideal current sources have output impedance of infinity and provide a specific current level. In actuality, these characteristics can only be approximated. In all implementations of current sources, they deviate from the ideal in at least two ways. The first is a parasitic parallel resistance which lowers the current source impedance from its ideal value of infinity. A second deviation is in the current level itself from the desired value because of current errors and biasing errors. This is particularly bothersome in bipolar current sources. Since the collector impedance of a bipolar transistor is inversely proportional to the current level, it is difficult to design a high impedance current source for high current levels with bipolar transistors. Also, it is difficult to set a precise current level since base current errors accumulate in bipolar transistors and drift as the bipolar transistor parameters drift with temperature changes.
A typical bipolar current source or current stack is illustrated in FIG. 1 including transistors Q1 and Q2 having their emitter-to-collector current paths connected between nodes N3 and N4. Their bases are connected to nodes N1 and N2. Nodes Nl, N2 and N4 are DC bias levels with the output current I-out being at node 3 and being represented by equation (1). EQU I.sub.out =ICQ2=IEQ1-IBQ1-IBQ2 (1)
The desired value for an ideal current source is IEQ1. The base currents for transistors Ql and Q2, namely IBQ1 and IBQ2, represent error terms.
The output impedance is dominated by the collector-base impedance of Q2, which is given by equation (2), EQU R=1/hobQ2 (2)
as the inverse of the small signal grounded base output admittance hobQ2. hob is a linear function of collector current level, so as the magnitude of the current source increases, its impedance decreases and deviates further from the ideal.
The prior art generally includes a sensor which provides a feedback to the input to cancel input bias and noise currents. A typical example in a differential bipolar amplifier is U.S. Pat. No. 4,639,684 to Laude. In operational amplifiers, the output impedance of a voltage gain stage has been increased by feeding back current from the output current stage as shown in U.S. Pat. No. 4,560,948 to Prentice and Cotreau.
Thus, it is an object of the present invention to reduce the current error produced by the base currents of the transistors Q1 and Q2 as well as minimizing the reduction of output impedance due to the collector impedance of the output transistor, and to make these improvements independent of the magnitude of the current source.
Another object of the present invention is to provide a high impedance, high accuracy current mirror.
A still further object of the present invention is to provide a high impedance, high accuracy differential to single-ended current converter or transconductance stage.
These and other objects of the invention are attained by providing a correction circuit connected to the bases of the first and second transistors for adding current to the output terminal as a function of the base current of the first and second transistors, and adding incremental current to the output terminal equal to those drawn by the second transistor to increase the output impedance to effectively cancel and replace the smaller output impedance of the second transistor with the higher impedance of a transistor in the correction circuit biased at lower current. This results in the first and second transistor forming a first current source having a first output current and first output impedance and a parallel second current source for providing a second output current smaller than the first output current and a second output impedance higher than the first output impedance at the output terminals. The correction circuit or second current source includes a third transistor having its emitter-collector current path connected in series between the bases of the first and second series connected transistors of the first current source and a fourth transistor having its emitter and collector connected between the base of the second or output transistor and the output terminal. The bases of the third and fourth transistor are connected to reference terminals. The third and fourth transistors have alpha's of approximately one.
This high impedance, high accuracy current source can be used as the output leg of a current mirror. The input leg would include appropriately connected transistors to bias the first and third transistors such that the sum of their collector currents is equal to the input current of the current mirror. Thus, the input leg would have a fifth transistor having its emitter-collector current path in the input leg and a sixth transistor having its emitter-collector path connected between the base of the fifth transistor and the emitter-collector path of the fifth transistor. The base of the third transistor is connected to the base of the sixth transistor and the emitter-collector path of the fifth transistor.
A high impedance, high accuracy differential current to single-ended current converter can also be produced using the high impedance, high accuracy current source and a modified high impedance, high accuracy current source. Each input would include a variable current source structured as the previously discussed high impedance, high accuracy current source with the current inputs being connected to the emitter-collector path of the first and second transistors. The output terminals are connected so as to subtract the currents of the two variable current sources. Preferably a high impedance, high accuracy current mirror connects the first current source to the second current source. As an alternative, the correction circuit of the second current source instead of adding current and impedance to the output of the second current source, subtracts the base current compensation from the output of the first current source and increases the output impedance to infinity with substituting a higher output impedance. This is done on the output leg of the current mirror.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.